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Comment by lm2s | Hacker News Reader

For the space that these diamond intersections use, wouldn't a roundabout also fit in that space and be a better option?

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Not necessarily.

Roundabouts lock up if the major flow of traffic has to go 270 degrees. (A left turn in an RHD country)

Diverging diamonds do their best at handling large left turn flows since that is the type of movement that has the most reduced number of conflicts. To get any lower you would need to start building flyover ramps.

I think it depends on the specific traffic expectations, a roundabout interrupts the traffic flow of everything that feeds into it... the double diamond can prioritize a more uneven flow in one direction (so might be better in cases where lots of people travel in one direction, like rush hour)

Might also be worth mentioning that Americans seem to be really bad at using roundabouts because they're not common in much of the country.

France uses roundabouts a lot and even has multi-lane roundabouts, the most famous one being around the Arc de Triomphe[1].

[1] https://maps.app.goo.gl/fSzgiWabGtwFhh4b8?g_st=ac

Paris under Anne Hidalgo has been undoing a lot of them because they are so hectic and pedestrian unfriendly.

Place de la Republique: https://www.lejdd.fr/Societe/La-place-de-la-Republique-se-re...

Same in the UK, I find it kind of funny that some North American transport planners are starting to put roundabouts everywhere at the same time as they're being ripped out in London.

They're still pretty much the default junction outside of towns and cities because they do work well but large roundabouts are no fun at all to cycle round

The type of roundabout being installed in America is generally one or two lanes wide, not some of the multilane monsters prevalent in the UK though.

I've also heard that traffic is so chaotic around the Arc de Triomphe that insurers always assume 50-50 fault for accidents within it.

That is an old-style traffic circle, not a roundabout, which is specially designed to improve safety, by requiring cars coming into the circle to approach at an angle, as opposed to 90 degrees, and requiring cars outside the roundabout to yield to cars inside. As someone who's actually driven around the Arc de Triomphe, it's quite dangerous - it is always congested, and has unmarked lanes inside the circle (between 4-5 depending on the whims of the drivers at that particular time). Modern roundabouts are a reaction to dangerous intersections like the Arc de Triomphe.

Like this:

https://i2-prod.examinerlive.co.uk/news/local-news/article24...

Having said that, as someone who has some historical familiarity with these sorts of intersections, but doesn't live in a country that has them, they are pretty scary too when encountering them only rarely.

For context, that's a very specific kind of roundabout that you find near the junction of a motorway (the M1) with a primary road (the A630). The roundabout is situated on the A-road and accessed from the motorway by a slip road. That way, motorway users who don't care about the junction can just keep cruising forwards and drive over the top of the roundabout. Nevertheless, drivers can approach from any direction and exit in any direction (including back the way they came). It doesn't look like this is the case in OP's article; I'm not sure how someone approaching from the left can continue in the same direction and drive off to the right.

Roundabouts generally look more like this in the UK (without any overpassing roads): https://www.google.com/maps/place/Howarth+Ln,+Rotherham/@53....

I picked that one because it matches the sort of intersection covered in TFA.

Also, the motorway passes under the roundabout, not over it.

Oh I see! Thanks for the clarification. I think my point still stands that the motorway is not interrupted in the case of the roundabout, whilst it is in the diamond intersection.

Nope, it's not :)

Look at TFA again. Just like the photo I linked to, the motorway (interstate in this case) passes under the 2 bridges.

The local version of this (NM14 & I25) has the interstate passing over on a single bridge, the double diamond is on the "ground level".

You're right; I hadn't scrolled down to the wide-view shot. I stand corrected.

I live in a Canadian province that has enthusiastically embraced roundabouts, so I’ve been able to get used to them for years. Their efficiency is very dependent on drivers being able to anticipate the actions of others.

These are things like watching the angle of the front wheels, the speed at which other cars are taking the intersection (faster = going straight through; slower = taking the turn), but also how much time it takes for a stopped car to start moving so you can judge whether you have time to enter the intersection.

This is a learned skill, and not everyone in the general population has the cognitive capacity to reliably pick up on these cues.

I’m not saying that I’m Nigel Mansell, but I genuinely think that the inability to pick up on these cues is why many are terrified and opposed to them.

Given that countries like the UK use roundabouts incredibly extensively, and they have never been shown to be a cause of increased accident rates, I suspect that your suspicion about this is probably wrong.

It depends on the country. Familiarity with roundabouts would be a necessity in the UK. In parts of the world where roundabouts are only recent, there are many drivers who are too old to “learn new tricks”.

I’ve got older family members that have self-excluded from driving to the provincial capital because the roundabouts are overwhelming.

I have older friends who self-exclude from driving at night. That doesn't make me conclude that night-time driving is inherently a problem (even though it is, based on statistics, more dangerous than driving around a roundabout).

It's asking for a lot of detailed attention in a lot of different places.

With right hand driving, I need to look left to consider if that traffic will be exiting or continuing. I need to look right to consider if that traffic has slowed and I need to slow. I also need to look hard left and hard right for pedestrians, who likely want to cross near the intersection (where it's most convenient) regardless of where traffic engineers are encouraging them to cross. Oh, and I better keep an eye on my mirrors, because I may need to take evasive action if the guy behind me isn't looking and I need to stop for traffic.

It's pretty hard to look at five different places at once.

I find it hard to believe that the diverging diamond interchange from the link is less scary than a roundabout. Roundabouts are all the same, from the smallest (https://www.google.com/maps/@40.2258039,-3.7709757,219m/data...) to the large (https://www.google.com/maps/@40.488386,-3.65609,436m/data=!3...), unless the designer felt the need to "improve". Arrive, yield, turn right, drive until your exit, leave.

Are you afraid of the inner lanes? Drive exclusively on the outer one, nobody cares. Do you need to go slower because you are in doubt? No problem, take even a second turn around if you need to get extra-sure.

I need to go through 16 roundabouts everyday, and after the 1000th (2 months) you gain familiarity enough.

What is this bullshit? https://www.google.com/maps/place/Madrid/@40.4965508,-3.6484... . That monstrosity, that you need to use carefully until you memorize it, is probably unique with all the mental toll that means. First time I used it, I made a bet on a lane and prayed that it went to where I wanted. A few kilometers south of that, you have this sane roundabout: https://www.google.com/maps/place/Madrid/@40.4297331,-3.5975..., doing the same 4-way joining in a more transparent and forgiving way.

Must the the Spanish influence :)

https://www.google.com/maps/@35.1052356,-106.6298892,1177m

>I find it hard to believe that the diverging diamond interchange from the link is less scary than a roundabout.

a (good) diverging diamond is really not scary. my first time driving through one i didn't even realize i was driving through one. if you came up to this intersection, would you be able to tell you were entering a diverging diamond? or would you think it was just a normal instersection of two one-way roads?

https://www.google.com/maps/@50.9043408,-114.0639993,3a,75y,...

Maybe it's that I never used one of them, so forgive me if I'm wrong: can I still go go the right it I missed the exit, and how? Can I do an U-turn and how? And finally, there are still traffic lights, and thus the potential to jump a red and t-bone someone.

I'm not really saying they are scary, but they are not less scary than a roundabout IMO.

Most roundabouts are the same, and then there are magic roundabouts.

https://www.youtube.com/watch?v=6OGvj7GZSIo

You don't have to predict other drivers on a diverging diamond.

What you can't see from the aerial view is that the road-level indicators, slope, and guard rails enforce the diverging part. They're quite simple.

Roundabouts feel adversarial. I don't like them.

They are not simple. You need traffic lights. You need road-level indicators, slopes and guard rails for them to work.

Meanwhile, I've seen roundabouts that are just a circle painted in the middle of the crossing, and that's it: everybody knows how to use it. See for example the main image of the linked article below.

USA people is so against roundabouts it hurts. According to Wikipedia, there are 160 diverging diamonds in the world, 150 of them in the USA. France built two in the 1970's, and it was so good that today they have... two. Meanwhile there are at least 300 roundabouts per million of habitants in Europe (France has 1,000 per million). Heck, even the US has at least 5,000 roundabouts. Some people call them "ugly", but even they say:

"Mélanie Cathelin, who runs a toy store on one of Abbeville’s main shopping streets, said she now finds traffic lights in other cities jarring, whether in neighboring Amiens or on a recent vacation in Florida."

"To cut down on the noise, traffic jams and fender benders occurring at one intersection, Mr. Dumont decided in 2010 to turn the troublesome spot into a roundabout. It solved the problems. Ten more traffic circles followed. In October, the city’s only remaining traffic light was sawed down."

So much for an adversarial structure...

src: https://www.nytimes.com/2019/12/25/world/europe/france-round...

the primary problem with roundabouts is their throughout. the only way to raise throughout is to increase the radius since roundabouts with more than 2 lanes don't really work. roundabouts are great for low speed roads with moderate congestion but as the speeds and number of cars increase, they make less sense

> roundabouts with more than 2 lanes don't really work.

Strange how there are so many of them in the UK then.

Where I am in the US, you run into them a lot. I find them easy to deal with, but I learned to drive on them.

But it is funny watching non-locals deal with them and I can see why they dislike them. They work great when only people use to them drive through them.

No. The entire point of these is to funnel the traffic onto and off the highway with out crossing any oncoming traffic, and to reduce the total number of traffic crossings overall.

Here’s a YouTube video that does a much better job of highlighting these benefits. https://youtu.be/A0sM6xVAY-A?si=qrk2ZFFYPA2-6rAS

Roundabouts don't have any traffic crossings, and sometimes don't even need traffic lights either so no one has to stop/give way

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